Zircon U-Pb dating of anatexis in high-grade metamorphic migmatite of the Bucaramanga gneiss in the Santander Massif: Implications for the Ordovician evolution of the Northern Andes of Colombia

Widespread anatexis was a regional response to the evolution of an Early Paleozoic orogen in NW South America that occurred during ca. 100 Myr. The orogen evolved during the subduction of oceanic lithosphere of the Iapetus Ocean beneath the western margin of Gondwana. This paper reviews the nature, timing, duration and conditions of anatexis for orthogneisses and migmatites in the Santander Massif (SM). These two lithologies are observed along the central part of the SM and share several features, such as similar age, common source rocks and clockwise P-T paths. Zircon ages of anatectic rocks in the SM range between ca. 483 and 411 Ma, suggesting prolonged crustal melting from the Late Cambrian to the Early Devonian during the Quetame orogeny in Colombia. Typically, a single sample analyzed for zircon U-Pb ages can reveal a range of ages spanning this ∼70 Myr period suggesting recycling of accessory phases from metamorphic rocks and early formed magmas. These results are consistent with findings of recent studies linking zircon ages with their composition that have determined the timing of prograde melting and retrograde melt crystallization and constrained the duration of the anatectic cycle. In some areas, this cycle becomes shorter (SM), whereas in other areas longer (Merida Andes). Orthogneisses and migmatites have a strong crustal signature and migration is controlled by low-angle foliation, leading to diffuse injection of complexes probably concentrated in the lower part of the Bucaramanga Gneiss.